When fluids, such as crude oil and natural gas, are produced it is often desirable to measure at downhole locations physical characteristics of the produced fluid(s) in order to ensure optimum production. Relevant characteristics are the pressure, temperature and composition of the fluid. Fluid composition monitoring is useful in reservoir formations where water or gas coning occurs around the well or wells through which crude oil is produced. In such reservoir formations it is therefore particularly relevant to continuously monitor the location(s) of the oil, gas and/or water interfaces at a variety of downhole locations.
Various methods exist to monitor fluid characteristics downhole.
U.S. Pat. No. 2,564,198 discloses a method wherein the in flow section of producing well is divided into a number of subsections by a removable well testing apparatus. The apparatus is equipped with a series of expandable packers.
The composition of the fluid that flows into each subsection is monitored by a fluid identifier unit which may measure the electrical conductivity of the produced fluid.
U.S. Pat. No. 5,132,903 discloses a method wherein a removable measuring sonde is lowered into the inflow region of an oil production well and a pad can be forced against the borehole wall to provide a sealed chamber from which fluid is evacuated by a pump and the properties of the thus withdrawn pore fluid(s) are measured. This method allows determination of the oil/water concentrations on the basis of a measurement of the dielectric properties of the produced fluids. Other dielectric well logging devices are disclosed in U.S. Pat. Nos. 2,973,477 and 4,677,386, German patent specification 2621142 and European patent specification 0111353.
A disadvantage of the known monitoring techniques is that use is made of measuring equipment which is temporarily lowered into the wells to perform the measurements and that these methods primarily measure characteristics of fluids that are flowing into the well.
An object of the present invention is to provide a method and device which enable a continuous downhole measurement of in-situ characteristics of the fluids in the pore spaces of the formation surrounding the wellbore.
Further objects of the present invention are to provide a downhole fluid monitoring method which can be carried out by means of a measuring device which can be easily installed at any location within a wellbore in such a way that it does not obstruct access to and/or production from lower parts of the well and which can be easily removed or replaced.